Phosducin, potential role in modulation of olfactory signaling

The choroid plexus as a sex hormone target: Functional implications

The choroid plexuses (CPs) are highly vascularized branched structures that protrude into the ventricles of the brain, and form a unique interface between the blood and the cerebrospinal fluid (CSF). In recent years, novel functions have been attributed to this tissue such as in immune and chemical surveillance of the central nervous system, brain development, adult neurogenesis and circadian rhythm regulation. Sex hormones (SH) are widely recognized as modulators in several neurodegenerative diseases, and there is evidence that estrogens and androgens regulate several fundamental biological functions in the CPs. Therefore, SH are likely to affect the composition of the CSF impacting on brain homeostasis. This review will look at implications of the CPs' sex-related specificities.

Mechanisms of regulation of olfactory transduction and adaptation in the olfactory cilium

Olfactory adaptation is a fundamental process for the functioning of the olfactory system, but the underlying mechanisms regulating its occurrence in intact olfactory sensory neurons (OSNs) are not fully understood. In this work, we have combined stochastic computational modeling and a systematic pharmacological study of different signaling pathways to investigate their impact during short-term adaptation (STA). We used odorant stimulation and electroolfactogram (EOG) recordings of the olfactory epithelium treated with pharmacological blockers to study the molecular mechanisms regulating the occurrence of adaptation in OSNs. EOG responses to paired-pulses of odorants showed that inhibition of phosphodiesterases (PDEs) and phosphatases enhanced the levels of STA in the olfactory epithelium, and this effect was mimicked by blocking vesicle exocytosis and reduced by blocking cyclic adenosine monophosphate (cAMP)-dependent protein kinase (PKA) and vesicle endocytosis. These results suggest that G-coupled receptors (GPCRs) cycling is involved with the occurrence of STA. To gain insights on the dynamical aspects of this process, we developed a stochastic computational model. The model consists of the olfactory transduction currents mediated by the cyclic nucleotide gated (CNG) channels and calcium ion (Ca²⁺)-activated chloride (CAC) channels, and the dynamics of their respective ligands, cAMP and Ca²⁺, and it simulates the EOG results obtained under different experimental conditions through changes in the amplitude and duration of cAMP and Ca²⁺ response, two second messengers implicated with STA occurrence. The model reproduced the experimental data for each pharmacological treatment and provided a mechanistic explanation for the action of GPCR cycling in the levels of second messengers modulating the levels of STA. All together, these experimental and theoretical results indicate the existence of a mechanism of regulation of STA by signaling pathways that control GPCR cycling and tune the levels of second messengers in OSNs, and not only by CNG channel desensitization as previously thought.

De novo transcriptomes of olfactory epithelium reveal the genes and pathways for spawning migration in japanese grenadier anchovy (Coilia nasus)

BACKGROUND

Coilia nasus (Japanese grenadier anchovy) undergoes spawning migration from the ocean to fresh water inland. Previous studies have suggested that anadromous fish use olfactory cues to perform successful migration to spawn. However, limited genomic information is available for C. nasus. To understand the molecular mechanisms of spawning migration, it is essential to identify the genes and pathways involved in the migratory behavior of C. nasus.

RESULTS

Using de novo transcriptome sequencing and assembly, we constructed two transcriptomes of the olfactory epithelium from wild anadromous and non-anadromous C. nasus. Over 178 million high-quality clean reads were generated using Illumina sequencing technology and assembled into 176,510 unigenes (mean length: 843 bp). About 51% (89,456) of the unigenes were functionally annotated using protein databases. Gene ontology analysis of the transcriptomes indicated gene enrichment not only in signal detection and transduction, but also in regulation and enzymatic activity. The potential genes and pathways involved in the migratory behavior were identified. In addition, simple sequence repeats and single nucleotide polymorphisms were analyzed to identify potential molecular markers.

CONCLUSION

We, for the first time, obtained high-quality de novo transcriptomes of C. nasus using a high-throughput sequencing approach. Our study lays the foundation for further investigation of C. nasus spawning migration and genome evolution.

A computational microscope focused on the sense of smell

In this article, we review studies of the protagonists of the perception of smell focusing on Odorant-Binding Proteins and Olfactory Receptors. We notably put forward studies performed by means of molecular modeling, generally combined with experimental data. Those works clearly emphasize that computational approaches are now a force to reckon with. In the future, they will certainly be more and more used, notably in the framework of a computational microscope meant to observe how the laws of physics govern the biomolecular systems originating our sense of smell.

Genetic basis of olfactory cognition: extremely high level of DNA sequence polymorphism in promoter regions of the human olfactory receptor genes revealed using the 1000 Genomes Project dataset

The molecular mechanism of olfactory cognition is very complicated. Olfactory cognition is initiated by olfactory receptor proteins (odorant receptors), which are activated by olfactory stimuli (ligands). Olfactory receptors are the initial player in the signal transduction cascade producing a nerve impulse, which is transmitted to the brain. The sensitivity to a particular ligand depends on the expression level of multiple proteins involved in the process of olfactory cognition: olfactory receptor proteins, proteins that participate in signal transduction cascade, etc. The expression level of each gene is controlled by its regulatory regions, and especially, by the promoter [a region of DNA about 100–1000 base pairs long located upstream of the transcription start site (TSS)]. We analyzed single nucleotide polymorphisms using human whole-genome data from the 1000 Genomes Project and revealed an extremely high level of single nucleotide polymorphisms in promoter regions of olfactory receptor genes and HLA genes. We hypothesized that the high level of polymorphisms in olfactory receptor promoters was responsible for the diversity in regulatory mechanisms controlling the expression levels of olfactory receptor proteins. Such diversity of regulatory mechanisms may cause the great variability of olfactory cognition of numerous environmental olfactory stimuli perceived by human beings (air pollutants, human body odors, odors in culinary etc.). In turn, this variability may provide a wide range of emotional and behavioral reactions related to the vast variety of olfactory stimuli.

Preferential binding of an odor within olfactory receptors: a precursor to receptor activation

Using computational methods, which allow mechanistic insights at a molecular level, we explored the olfactory receptor (OR)-odor interactions for 2 mouse ORs, S79 and S86. Both ORs have been previously experimentally, functionally characterized. The odors used were mostly carboxylic acids, which differed in chain length, substituents on the primary carbon atom-chain and degree of unsaturation. These odors elicited varied activation responses from both ORs. Our studies revealed that both receptors have 2 distinct binding sites. Preferential binding in 1 of the 2 sites is correlated with OR activation. The activating odorants: nonanedioic acid, heptanoic acid, and octanoic acid for OR S79 and nonanoic acid for OR S86 preferentially bind in the region bound by transmembranes (TMs [helical domains]) III, IV, V, and VI. The non excitatory odorants heptanol for S79 and heptanoic acid for S86 showed a greater likelihood of binding in the region bound by TMs I, II, III, and VII. Nanosecond-scale molecular dynamics simulations of the physiologically relevant conditions of docked OR-odorant complexes enabled us to quantitatively assess the roles of individual OR amino acids in odor binding. Amino acid-odorant contact maps and distance determinations over the course of the simulations lend support to our conclusions.

Phosducin interacts with the G-protein βγ-dimer of ciliate protozoanBlepharisma japonicumupon illumination

Immunological techniques and high-resolution FRET analysis were employed to investigate the in vivo colocalization and interaction of phosducin(Pdc) with the βγ-subunits of G-protein (Gβγ) in the ciliate Blepharisma japonicum. Immunological techniques revealed that illumination of cells resulted in a decrease in phosphorylation levels of Pdc and its colocalization with Gβγ. The observed light-induced Pdc dephosphorylation was also accompanied by significant enhancement of Gβγ binding by this molecule. Possible formation of the Pdc–Gβγ complex in cells exposed to light was corroborated by FRET between these proteins. Treatment of cells with okadaic acid, an inhibitor of phosphatase activity, entirely prevented Pdc dephosphorylation by light, colocalization of this phosphoprotein with Gβγ and generation of the Pdc–Gβγ complex. Cell fractionation and immunoblotting revealed that in cells exposed to light, the formation of Pdc–Gβγ complex and its translocation into the cytoplasm occur simultaneously with a change in the gel migration of Gβ. Moreover, a 33 kDa immunoanalog of 14-3-3 protein was identified and we showed that this protein is bound by phosphorylated Pdc in a cell adapted to darkness. The results of this study provide additional detailed characterization of the functional properties of the ciliate Pdc. The likely functional role of Pdc in Blepharisma is discussed.

Function of phosducin-like proteins in G protein signaling and chaperone-assisted protein folding

Members of the phosducin gene family were initially proposed to act as down-regulators of G protein signaling by binding G protein betagamma dimers (Gbetagamma) and inhibiting their ability to interact with G protein alpha subunits (Galpha) and effectors. However, recent findings have over-turned this hypothesis by showing that most members of the phosducin family act as co-chaperones with the cytosolic chaperonin complex (CCT) to assist in the folding of a variety of proteins from their nascent polypeptides. In fact rather than inhibiting G protein pathways, phosducin-like protein 1 (PhLP1) has been shown to be essential for G protein signaling by catalyzing the folding and assembly of the Gbetagamma dimer. PhLP2 and PhLP3 have no role in G protein signaling, but they appear to assist in the folding of proteins essential in regulating cell cycle progression as well as actin and tubulin. Phosducin itself is the only family member that does not participate with CCT in protein folding, but it is believed to have a specific role in visual signal transduction to chaperone Gbetagamma subunits as they translocate to and from the outer and inner segments of photoreceptor cells during light-adaptation.

Photosensory transduction in unicellular eukaryotes: A comparison between related ciliates Blepharisma japonicum and Stentor coeruleus and photoreceptor cells of higher organisms

Blepharisma japonicum and Stentor coeruleus are related ciliates, conspicuous by their photosensitivity. They are capable of avoiding illuminated areas in the surrounding medium, gathering exclusively in most shaded places (photodispersal). Such behaviour results mainly from motile photophobic response occurring in ciliates. This light-avoiding response is observed during a relatively rapid increase in illumination intensity (light stimulus) and consists of cessation of cell movement, a period of backward movement (ciliary reversal), followed by a forward swimming, usually in a new direction. The photosensitivity of ciliates is ascribed to their photoreceptor system, composed of pigment granules, containing the endogenous photoreceptor -- blepharismin in Blepharisma japonicum, and stentorin in Stentor coeruleus. A light stimulus, applied to both ciliates activates specific stimulus transduction processes leading to the electrical changes at the plasma membrane, correlated with a ciliary reversal during photophobic response. These data indicate that both ciliates Blepharisma japonicum and Stentor coeruleus, the lower eukaryotes, are capable of transducing the perceived light stimuli in a manner taking place in some photoreceptor cells of higher eukaryotes. Similarities and differences concerning particular stages of light transduction in eukaryotes at different evolutional levels are discussed in this article.

Motif-based construction of a functional map for mammalian olfactory receptors

We applied an automatic and unsupervised system to a nearly complete database of mammalian odor receptor genes. The generated motifs and gene classification were subjected to extensive and systematic downstream analysis to obtain biological insights. Two major results from this analysis were: (1) a map of sequence motifs that may correlate with function and (2) the corresponding receptor classes in which members of each class are likely to share specific functions. We have discovered motifs that have been implicated in structural integrity and posttranslational modification, as well as motifs very likely to be directly involved in ligand binding. We further propose a combinatorial molecular hypothesis, based on unique combinations of the observed motifs, that provides a foundation for understanding the generation of a large number of ligand binding sites.

Regulation of angiotensin II-induced G protein signaling by phosducin-like protein

Phosducin-like protein (PhLP) is a broadly expressed member of the phosducin (Pd) family of G protein betagamma subunit (Gbetagamma)-binding proteins. Though PhLP has been shown to bind Gbetagamma in vitro, little is known about its physiological function. In the present study, the effect of PhLP on angiotensin II (Ang II) signaling was measured in Chinese hamster ovary cells expressing the type 1 Ang II receptor and various amounts of PhLP. Up to 3.6-fold overexpression of PhLP had no effect on Ang II-stimulated inositol trisphosphate (IP(3)) formation, whereas further increases caused an abrupt decrease in IP(3) production with half-maximal inhibition occurring at 6-fold PhLP overexpression. This threshold level for inhibition corresponds to the cellular concentration of cytosolic chaperonin complex, a recently described binding partner that preferentially binds PhLP over Gbetagamma. Results of pertussis toxin sensitivity, GTPgammaS binding, and immunoprecipitation experiments suggest that PhLP inhibits phospholipase Cbeta activation by dual mechanisms: (i) steric blockage of Gbetagamma activation of PLCbeta and (ii) interference with Gbetagamma-dependent cycling of G(q)alpha by the receptor. These results suggest that G protein signaling may be regulated through controlling the cellular concentration of free PhLP by inducing its expression or by regulating its binding to the chaperonin.

G proteins and pheromone signaling

All cells have the capacity to respond to chemical and sensory stimuli. Central to many such signaling pathways is the heterotrimeric G protein, which transmits a signal from cell surface receptors to intracellular effectors. Recent studies using the yeast Saccharomyces cerevisiae have produced important advances in our understanding of G protein activation and inactivation. This review focuses on the mechanisms by which G proteins transmit a signal from peptide pheromone receptors to the mating response in yeast and how mechanisms elucidated in yeast can provide insights to signaling events in more complex organisms.

Glycosylated phosducin-like protein long regulates opioid receptor function in mouse brain

Phosducin (Phd), a protein that in retina regulates rhodopsin desensitization by controlling the activity of Gt beta gamma-dependent G-protein-coupled receptor kinases (GRKs), is present in very low levels in the CNS of mammals. However, this tissue contains proteins of related sequence and function. This paper reports the presence of N-glycosylated phosducin-like protein long (PhLP(L)) in all structures of mouse CNS, mainly in synaptic plasma membranes and associated with G beta subunits and 14-3-3 proteins. To analyze the role PhLP(L) in opioid receptor desensitization, its expression was reduced by the use of antisense oligodeoxynucleotides (ODNs). The antinociception induced by morphine, [D-Ala(2), N-MePhe(4),Gly-ol(5)]-enkephalin (DAMGO), beta-endorphin, [D-Ala(2)]deltorphin II, [D-Pen(2,5)]-enkephalin (DPDPE) or clonidine in the tail-flick test was reduced in PhLP(L)-knock-down mice. A single intracerebroventricular (icv)-ED(80) analgesic dose of morphine gave rise to acute tolerance that lasted for 4 days, but which was prevented or reversed by icv-injection of myristoylated (myr(+)) G(i2)alpha subunits. PhLP(L) knock-down brought about a myr(+)-G(i2)alpha subunit-insensitive acute tolerance to morphine that was still present after 8 days. It also diminished the specific binding of (125)I-Tyr(27)-beta-endorphin-(1-31) (human) to mouse periaqueductal gray matter membranes. After being exposed to chronic morphine treatment, post-dependent mice required about 10 days for complete recovery of morphine antinociception. The impairment of PhLP(L) extended this period beyond 17 days. It is concluded that PhLP(L) knock-down facilitates desensitization and uncoupling of opioid receptors.

Modulation of the G protein regulator phosducin by Ca2+/calmodulin-dependent protein kinase II phosphorylation and 14-3-3 protein binding

Phototransduction is a canonical G protein-mediated cascade of retinal photoreceptor cells that transforms photons into neural responses. Phosducin (Pd) is a Gbetagamma-binding protein that is highly expressed in photoreceptors. Pd is phosphorylated in dark-adapted retina and is dephosphorylated in response to light. Dephosphorylated Pd binds Gbetagamma with high affinity and inhibits the interaction of Gbetagamma with Galpha or other effectors, whereas phosphorylated Pd does not. These results have led to the hypothesis that Pd down-regulates the light response. Consequently, it is important to understand the mechanisms of regulation of Pd phosphorylation. We have previously shown that phosphorylation of Pd by cAMP-dependent protein kinase moderately inhibits its association with Gbetagamma. In this study, we report that Pd was rapidly phosphorylated by Ca(2+)/calmodulin-dependent kinase II, resulting in 100-fold greater inhibition of Gbetagamma binding than cAMP-dependent protein kinase phosphorylation. Furthermore, Pd phosphorylation by Ca(2+)/calmodulin-dependent kinase II at Ser-54 and Ser-73 led to binding of the phosphoserine-binding protein 14-3-3. Importantly, in vivo decreases in Ca(2+) concentration blocked the interaction of Pd with 14-3-3, indicating that Ca(2+) controls the phosphorylation state of Ser-54 and Ser-73 in vivo. These results are consistent with a role for Pd in Ca(2+)-dependent light adaptation processes in photoreceptor cells and also suggest other possible physiological functions.

The pharmacology of phosducin

The discovery of phosducin (Phd) in photoreceptor cells of the retina and the further identification of phosducin-like proteins (PhdLP) emphasizes the existence of a family of proteins characterized as cytosolic regulators of G protein functions. The individual members represent phosphoproteins with distinct tissue distributions whose highest concentrations were in the retina and the pineal gland, while lower levels were reported for tissues such as liver, spleen, striated muscle, and the brain. Several functions of Phd and PhdLP have been suggested, but their most important ability appears to be their high affinity sequestration with G betagamma subunits of heterotrimeric G proteins. This finding suggests that neutralization of G betagamma by Phd effectively impedes G protein-mediated signal transmission, since G alpha cannot reassemble with G betagamma to provide a functional G protein trimer (G alphabetagamma). Thus, it is the scavenger quality of Phd that is hypothesized to diminish intracellular communication simply by reducing the number of G proteins. An additional important function of Phd relates to the inhibition of G alpha subunits' inherent GTPase. The ability of Phd to directly bind G alpha subunits is probably of minor significance as the affinity between both proteins is low. In general, similar mechanisms have been reported for PhdLPs. In the majority of investigations concerning the interference of Phd with physiological mechanisms, the dark/light adaptation of retinal photoreceptor cells has been the most frequently studied aspect of Phd. More recently, Phd was associated with the adenylyl cyclase of olfactory cilia, as in the presence of the phosphoprotein an increased concentration of cAMP is observed. This finding is in line with the experimental outcome of permanent cell lines transfected to overexpress Phd, which exhibit sensitization to excitatory acting PGE(1), and isoproterenol, respectively. Furthermore, Phd was found to effectively slow down the mechanism of internalization of G protein-coupled opioid receptors. Pathophysiological processes associated with Phd were found for certain eye diseases. Experimental evidence suggests the development of retinal inflammation as a consequence of an autoimmunization process triggered by Phd or shorter fragments thereof. Thus, our present knowledge regarding the functions of members of the Phd family is limited currently to their control of G protein-mediated intracellular signal transmission, the process of endocytosis, and certain autoimmune diseases of the uvea and the pineal gland. However, recent information regarding the presence of certain members of the Phd family in the cell nucleus may bear new insights into the function of these compounds.

Phosphorylation of phosducin and phosducin-like protein by G protein-coupled receptor kinase 2

G protein-coupled receptor kinase 2 (GRK2) is able to phosphorylate a variety of agonist-occupied G protein-coupled receptors (GPCR) and plays an important role in GPCR modulation. However, recent studies suggest additional cellular functions for GRK2. Phosducin and phosducin-like protein (PhLP) are cytosolic proteins that bind Gbetagamma subunits and act as regulators of G-protein signaling. In this report, we identify phosducin and PhLP as novel GRK2 substrates. The phosphorylation of purified phosducin and PhLP by recombinant GRK2 proceeds rapidly and stoichiometrically (0.82 +/- 0.1 and 0.83 +/- 0.09 mol of P(i)/mol of protein, respectively). The phosphorylation reactions exhibit apparent K(m) values in the range of 40-100 nm, strongly suggesting that both proteins could be endogenous targets for GRK2 activity. Our data show that the site of phosducin phosphorylation by GRK2 is different and independent from that previously reported for the cAMP-dependent protein kinase. Analysis of GRK2 phosphorylation of a variety of deletion mutants of phosducin and PhLP indicates that the critical region for GRK2 phosphorylation is localized in the C-terminal domain of both phosducin and PhLP (between residues 204 and 245 and 195 and 218, respectively). This region is important for the interaction of these proteins with G beta gamma subunits. Phosphorylation of phosducin by GRK2 markedly reduces its G beta gamma binding ability, suggesting that GRK2 may modulate the activity of the phosducin protein family by disrupting this interaction. The identification of phosducin and PhLP as new substrates for GRK2 further expands the cellular roles of this kinase and suggests new mechanisms for modulating GPCR signal transduction.

Modulation of odor-induced increases in [Ca(2+)](i) by inhibitors of protein kinases A and C in rat and human olfactory receptor neurons

Protein kinases A and C have been postulated to exert multiple effects on different elements of signal transduction pathways in olfactory receptor neurons. However, little is known about the modulation of olfactory responses by protein kinases in intact olfactory receptor neurons. To further elucidate the details of the modulation of odorant responsiveness by these protein kinases, we investigated the action of two protein kinase inhibitors: H89, an inhibitor of protein kinase A, and N-myristoylated EGF receptor, an inhibitor of protein kinase C, on odorant responsiveness in intact olfactory neurons. We isolated individual olfactory neurons from the adult human and rat olfactory epithelium and measured responses of the isolated cells to odorants or biochemical activators that have been shown to initiate cyclic AMP or inositol 1,4,5-trisphospate production in biochemical preparations. We employed calcium imaging techniques to measure odor-elicited changes in intracellular calcium that occur over several seconds. In human olfactory receptor neurons, the protein kinase A and C inhibitors affected the responses to different sets of odorants. In rats, however, the protein kinase C inhibitor affected responses to all odorants, while the protein kinase A inhibitor had no effect. In both species, the effect of inhibition of protein kinases was to enhance the elevation and block termination of intracellular calcium levels elicited by odorants. Our results show that protein kinases A and C may modulate odorant responses of olfactory neurons by regulating calcium fluxes that occur several seconds after odorant stimulation. The effects of protein kinase C inhibition are different in rat and human olfactory neurons, indicating that species differences are an important consideration when applying data from animal studies to apply to humans.

A possible role for caveolin as a signaling organizer in olfactory sensory membranes

Fast kinetics and sensitivity of olfactory signaling raise the question of whether the participating proteins may be associated in supramolecular transduction complexes. We found evidence that caveolin proteins could play an important role in organizing signaling elements in olfactory sensory neurons. Western blot analysis indicated that caveolins are highly enriched in olfactory sensory membranes, where they co-localize in detergent-insoluble complexes with key components of the signaling pathways. Furthermore, the results of immunoprecipitation experiments suggest that G proteins and effector enzyme form preassembled subcellular complexes with caveolins. Since anti-caveolin antibodies and synthetic peptides derived from the scaffolding domains of caveolin-1 and caveolin-2 effectively attenuated second messenger responses in sensory cilia preparations in a characteristic manner, the data led to the suggestion that caveolins could mediate the assembly of signaling complexes within specialized membrane microdomains of olfactory sensory neurons.

Functional analysis of Plp1 and Plp2, two homologues of phosducin in yeast

Mammalian phosducins are known to bind G protein betagamma subunits in vitro, and are postulated to regulate their signaling function in vivo. Here we describe two homologues of phosducin in yeast, called PLP1 and PLP2. Both gene products were cloned, expressed, and purified as glutathione S-transferase fusions. Of the two isoforms, Plp1 bound most preferentially to Gbetagamma. Binding was enhanced by pheromone stimulation and by the addition of GTPgammaS, conditions that favor dissociation of Gbetagamma from Galpha. Gene disruption mutants and gene overexpression plasmids were prepared and analyzed for changes in signaling and nonsignaling phenotypes. Haploid spore products bearing the plp2Delta mutant failed to grow, suggesting that PLP2 is an essential gene. Cell viability was not restored by a mutation in STE7 that blocks signaling downstream of the G protein. Haploid products bearing the plp1Delta mutant were viable and exhibited a 6-7% increase in pheromone-mediated gene induction. Cells overexpressing PLP1 or PLP2 exhibited a 70-80% decrease in gene induction but no change in pheromone-mediated growth arrest. These data indicate that phosducin can selectively regulate early signaling events following pheromone stimulation and has an essential role in cell growth independent of its regulatory role in cell signaling.

Cyclic adenosine monophosphate signaling in the rat vomeronasal organ: role of an adenylyl cyclase type VI

The present study indicates that male rat urinary components in female rat vomeronasal organ microvillar preparations not only induce a rapid and transient IP(3) signal, but in addition, the level of cAMP decreases with a delayed and sustained time course. This decrease seems to be a consequence of the preceding activation of the phosphoinositol pathway rather than the result of an enhanced phosphodiesterase activity or an inhibition of adenylyl cyclase (AC) via Galpha(i) or Galpha(o). This notion is supported by the finding that activation of the endogenous protein kinase C suppresses basal as well as forskolin-induced cAMP formation. Furthermore, it was observed that elevated levels of calcium inhibit cAMP formation in rat VNO microvillar preparations. These properties of cAMP signaling in the VNO of rats may be mediated by a calcium- and protein kinase C-inhibited AC VI subtype, which is localized in microvillar preparations of the VNO.

Cellular and molecular constituents of olfactory sensation in vertebrates

Since the discovery of odorant-activated adenylate cyclase in the olfactory receptor cilia, research into the olfactory perception of vertebrates has rapidly expanded. Recent studies have shown how the odor discrimination starts at the receptor level: each of 700-1000 types of the olfactory neurons in the neural olfactory epithelium contains a single type of odor receptor protein. Although the receptors have relatively low specific affinities for odorants, excitation of different types of receptors forms an excitation pattern specific to each odorant in the glomerular layer of the olfactory bulb. It was demonstrated that adenosine 3',5'-cyclic monophosphate (cAMP) is very likely the sole second messenger for olfactory transduction. It was also demonstrated that the affinity of the cyclic nucleotide-gated channel for cAMP regulated by Ca(2+)/calmodulin is solely responsible for the adaptation of the cell. However, many other regulatory components were found in the transduction cascade. Regulated by Ca(2+) and/or the protein-phosphorylation, many of them may serve for the adaptation of the cell, probably on a longer time scale. It may be important to consider the resensitization as a part of this adaptation, as well as to collect kinetic data of each reaction to gain further insight into the olfactory mechanism.

Identification of Gbetagamma binding sites in the third intracellular loop of the M(3)-muscarinic receptor and their role in receptor regulation

Gbetagamma binds directly to the third intracellular (i3) loop subdomain of the M(3)-muscarinic receptor (MR). In this report, we identified the Gbetagamma binding motif and G-protein-coupled receptor kinase (GRK2) phosphorylation sites in the M(3)-MR i3 loop via a strategy of deletional and site-directed mutagenesis. The Gbetagamma binding domain was localized to Cys(289)-His(330) within the M(3)-MR-Arg(252)-Gln(490) i3 loop, and the binding properties (affinity, influence of ionic strength) of the M(3)-MR-Cys(289)-His(330) i3 loop subdomain were similar to those observed for the entire i3 loop. Site-directed mutagenesis of the M(3)-MR-Cys(289)-His(330) i3 loop subdomain indicated that Phe(312), Phe(314), and a negatively charged region (Glu(324)-Asp(329)) were required for interaction with Gbetagamma. Generation of the full-length M(3)-MR-Arg(252)-Gln(490) i3 peptides containing the F312A mutation were also deficient in Gbetagamma binding and exhibited a reduced capacity for phosphorylation by GRK2. A similar, parallel strategy resulted in identification of major residues ((331)SSS(333) and (348)SASS(351)) phosphorylated by GRK2, which were just downstream of the Gbetagamma binding motif. Full-length M(3)-MR constructs lacking the 42-amino acid Gbetagamma binding domain (Cys(289)-His(330)) or containing the F312A mutation exhibited ligand recognition properties similar to wild type receptor and also effectively mediated agonist-induced increases in intracellular calcium following receptor expression in Chinese hamster ovary and/or COS 7 cells. However, the M(3)-MRDeltaCys(289)-His(330) and M(3)-MR(F312A) constructs were deficient in agonist-induced sequestration, indicating a key role for the Gbetagamma-M(3)-MR i3 loop interaction in receptor regulation and signal processing.

Identification of bdm-1, a gene involved in G protein beta-subunit function and alpha-subunit accumulation

Targeted disruption of Galpha and Gbeta genes has established the requirement of an intact G protein signaling pathway for optimal execution of several important physiological processes, including pathogenesis, in the chestnut blight fungus Cryphonectria parasitica. We now report the identification of a G protein signal transduction component, beta disruption mimic factor-1, BDM-1. Disruption of the corresponding gene, bdm-1, resulted in a phenotype indistinguishable from that previously observed after disruption of the Gbeta subunit gene, cpgb-1. The BDM-1 deduced amino acid sequence contained several significant clusters of identity with mammalian phosducin, including a domain corresponding to a highly conserved 11-amino acid stretch that has been implicated in binding to the Gbetagamma dimer and two regions of defined Gbeta/phosducin contact points. Unlike the negative regulatory function proposed for mammalian phosducin, the genetic data presented in this report suggest that BDM-1 is required for or facilitates Gbeta function. Moreover, disruption of either bdm-1 or cpgb-1 resulted in a significant, posttranscriptional reduction in the accumulation of CPG-1, a key Galpha subunit required for a range of vital physiological processes.

Lobster G-protein coupled receptor kinase that associates with membranes and G(beta) in response to odorants and neurotransmitters

A cDNA clone (lobGRK2) encoding a protein of 690 amino acids with significant similarity to the GRK2 subfamily of G-protein coupled receptor kinases was isolated. lobGRK2 was widely expressed as a 9-kb major transcript and a protein of 80 kDa. It was most abundant in the brain and the olfactory organ but was absent in the eye/eyestalk. Immunocytochemistry revealed lobGRK2 immunoreactivity in the outer dendritic segments of the olfactory receptor neurons, the site of olfactory transduction. LobGRK2 immunoreactivity was observed in most neuronal structures in the brain, although with varying intensity. It was strongest in neuropil, especially the olfactory and accessory lobes but was also detectable in neuronal cell bodies. Stimulation of brain homogenates with a mixture of neurotransmitters increased the association of lobGRK2 with membranes and with G(beta). Similarly, stimulation of olfactory dendrite homogenates with an odorant mixture caused lobGRK2 to associate with G(beta). These results support the conclusion that lobGRK2 responds to odorants and to neurotransmitters and may act to initiate desensitization by phosphorylating G-protein-coupled receptors in the olfactory organ and the brain, respectively.

A molecular mechanism for the phosphorylation-dependent regulation of heterotrimeric G proteins by phosducin

Visual signal transduction is a nearly noise-free process that is exquisitely well regulated over a wide dynamic range of light intensity. A key component in dark/light adaptation is phosducin, a phosphorylatable protein that modulates the amount of transducin heterotrimer (Gt alpha beta gamma) available through sequestration of the beta gamma subunits (Gt beta gamma). The structure of the phosphophosducin/Gt beta gamma complex combined with mutational and biophysical analysis provides a stereochemical mechanism for the regulation of the phosducin-Gt beta gamma interaction. Phosphorylation of serine 73 causes an order-to-disorder transition of a 20-residue stretch, including the phosphorylation site, by disrupting a helix-capping motif. This transition disrupts phosducin's interface with Gt beta gamma, leading to the release of unencumbered Gt beta gamma, which reassociates with the membrane and Gt alpha to form a signaling-competent Gt alpha beta gamma heterotrimer.

The variable and conserved interfaces of modeled olfactory receptor proteins

The accumulation of hundreds of olfactory receptor (OR) sequences, along with the recent availability of detailed models of other G-protein-coupled receptors, allows us to analyze the OR amino acid variability patterns in a structural context. A Fourier analysis of 197 multiply aligned olfactory receptor sequences showed an alpha-helical periodicity in the variability profile. This was particularly pronounced in the more variable transmembranal segments 3, 4, and 5. Rhodopsin-based homology modeling demonstrated that the inferred variable helical faces largely point to the interior of the receptor barrel. We propose that a set of 17 hypervariable residues, which point to the barrel interior and are more extracellularly disposed, constitute the odorant complementarity determining regions. While 12 of these residues coincide with established ligand-binding contact positions in other G-protein-coupled receptors, the rest are suggested to form an olfactory-unique aspect of the binding pocket. Highly conserved olfactory receptor-specific sequence motifs, found in the second and third intracellular loops, may comprise the G-protein recognition epitope. The prediction of olfactory receptor functional sites provides concrete suggestions of site-directed mutagenesis experiments for altering ligand and G-protein specificity.

Structural features of heterotrimeric G-protein-coupled receptors and their modulatory proteins

Over the past 20 years, the general mechanism for signaling through 7-transmembrane helix receptors coupled to GTP hydrolysis has been worked out. Although similar in overall organization, subtype variability and subcellular localization of components have built in considerable signaling specificity. Atomic resolution structures for many of the components have delineated the domain organization of these complex proteins and have given physical form to the idea of subtype specificity. This review describes what is known about the physical structures of the 7-transmembrane helix receptors, the heterotrimeric GTP binding coupling proteins, the adenylate cyclase and phospholipase C effector proteins, and signaling modulatory proteins, such as arrestin, phosducin, recoverin-type myristoyl switch proteins, and the pleckstrin homology domain of G-protein receptor kinase-2. These images allow experimenters to contemplate the details of the supramolecular organization of the multiprotein complexes involved in the transmission of signals across the cellular lipid bilayer.

Molecular mechanisms and regulation of insulin exocytosis as a paradigm of endocrine secretion

Secretion of the peptide hormone insulin from pancreatic beta cells constitutes an important step in the regulation of body homeostasis. Insulin is stored in large dense core vesicles and released by exocytosis, a multistage process involving transport of vesicles to the plasma membrane, their docking, priming and finally their fusion with the plasma membrane. Some of the protein components necessary for this process have been identified in beta cells. The export of potent and potentially harmful substances has to be tightly controlled. The secretory response in pancreatic beta cells requires the concerted action of nutrients together with enteric hormones and neurotransmitters acting on G-protein coupled receptors. It is well established that glucose and other metabolizable nutrients depolarize the beta-cell membrane and the ensuing Ca2+ influx through voltage-dependent channels constitutes a main stimulus for insulin exocytosis. Theoretical considerations and recent observations suggest in addition an organizing role for the Ca2+ channel similar to neurotransmission. A second regulatory control on exocytosis is exerted by monomeric and heterotrimeric G-proteins. The monomeric GTPase Rab3A controls insulin secretion through cycling between a guanosine triphosphate liganded vesicle-bound form and a guanosine diphosphate liganded, cytosolic form. The effect of neurohormones is transduced by the heterotrimeric GTPases. Whereas pertussis-toxin sensitive alpha-subunits exert direct inhibition at the level of exocytosis, the Gbeta gamma-subunits are required for stimulation. It is possible that these GTPases exert immediate regulation, while protein kinases and phosphatases may modulate long-term adaptation at the exocytotic machinery itself. The molecular nature of their activators and effectors still await identification. Insights into the progression of the exocytotic vesicle from docking to fusion and how these processes are precisely regulated by proteins and second messengers may provide the basis for new therapeutic principles.

PhLPs and PhLOPs in the phosducin family of G beta gamma binding proteins

In this study, we identify new isoforms of the retinal phosducin and investigate the expression of the phosducin family, showing that an isoform, PhLP1, has sequence homology with Phd and Gbeta gamma binding capability, whereas two isoforms (phosducin-like orphan proteins, PhLOPs) share sequence homology with Phd but fail to bind Gbeta gamma. Original identification of PhLP1 and the PhLOPs was from a human retina cDNA library, using a PCR product for library hybridization screening that contained a predicted functional epitope domain. The screen identified Phd and three related, but distinct, recombinants (PhLP1, PhLOP1, and PhLOP2). By RT-PCR, all isoforms are expressed in either retina or forskolin-stimulated Y79 retinoblastoma cells; however, the new isoforms are below the level of detection on Northern blot analysis. The predicted amino acid translation of each homologue revealed major differences, arising from either splice variants or gene duplication of Phd. To test the functional interaction of all phosducin isoforms with Gbeta gamma in vitro, a glutathione S-transferase (GST) fusion protein was developed for each member. Biochemical interaction with purified retinal transducin Gbeta gamma was verified for GST-Phd and demonstrated for GST-PhLP1; however, neither GST-PhLOP1 nor GST-PhLOP2 bound Gbeta gamma. Comparable results were observed when the GST-phosducin fusion proteins selectively sequestered Gbeta gammas from retinal extracts or when functional Gbeta gamma interactions were assessed using surface plasmon resonance technology. Phosducin and its isoforms are widely distributed in body tissues where they may participate in signal transduction pathways. Phd and PhLP1 possess an 11-amino acid conserved epitope domain (TGPKGVINDWR) that controls the high-affinity binding of Gbeta gamma; these isoforms are implicated in the G-protein signaling pathway. The phosducin-like orphan proteins (PhLOPs) fail to bind Gbeta gamma, suggesting that the PhLOP isoforms may participate in still unidentified signaling pathways.

G protein-coupled receptor kinases

G protein-coupled receptor kinases (GRKs) constitute a family of six mammalian serine/threonine protein kinases that phosphorylate agonist-bound, or activated, G protein-coupled receptors (GPCRs) as their primary substrates. GRK-mediated receptor phosphorylation rapidly initiates profound impairment of receptor signaling, or desensitization. This review focuses on the regulation of GRK activity by a variety of allosteric and other factors: agonist-stimulated GPCRs, beta gamma subunits of heterotrimeric GTP-binding proteins, phospholipid cofactors, the calcium-binding proteins calmodulin and recoverin, posttranslational isoprenylation and palmitoylation, autophosphorylation, and protein kinase C-mediated GRK phosphorylation. Studies employing recombinant, purified proteins, cell culture, and transgenic animal models attest to the general importance of GRKs in regulating a vast array of GPCRs both in vitro and in vivo.

Characterization of canine photoreceptor phosducin cDNA and identification of a sequence variant in dogs with photoreceptor dysplasia

Photoreceptor dysplasia (pd) is an autosomal recessive disease of miniature schnauzer dogs causing retinal degeneration. The disease is a homologue of retinitis pigmentosa, a group of genetically heterogeneous diseases, causing blindness in humans. A subtraction library was prepared from retinas of pd affected and age-matched normal control dogs to isolate de novo candidate genes for further examination. From the subtraction library, cDNA for phosducin (PDC), a member of the phototransduction pathway, was isolated as a transcript expressed at a higher level in the affected retina. First, the normal canine PDC cDNA was characterized to evaluate the PDC gene in the pd-affected retina. The characterized region of normal PDC cDNA spans 1258 nucleotides (nt) that include 738 nt of coding sequence predicted to encode a protein (Mr=28 209) of 245 amino acids (aa). Over the coding region, PDC shares 86-95% nt sequence identity and 90-95% identity in the deduced aa sequence with homologous mammalian sequences. A major transcript (1.9 kb) was observed only in retina by Northern analysis, but low levels of transcript were detected in brain, liver and kidney by reverse transcription and polymerase chain reaction. Retinal immunocytochemistry showed that PDC was detected only in rod photoreceptors, mainly in the inner segment and perinuclear region. By Northern blot analysis, increased PDC expression was observed in pre-degenerate affected retina relative to the age-matched normal. In pd- affected miniature schnauzer pedigree, a missense mutation was detected in codon 82 (CGA to GGA) that would create a non-conservative substitution (Arg to Gly) in close vicinity to the residue (Glu 85) which directly interacts with the betagamma-subunits of transducin. Only pd-affected dogs were found to be homozygous for the mutant allele, and none among 48 dogs tested from 20 other dog breeds had this allele, suggesting that the mutation could be causally associated with pd in miniature schnauzers. However, since some affected dogs are heterozygous for the mutant allele, and some are homozygous for the wild-type allele, this putative PDC missense mutation, if it is indeed a disease causing mutation, does not account entirely for the genetics of inherited retinal degeneration in the miniature schnauzer breed.

Translocation of phosducin in living neuroblastoma x glioma hybrid cells (NG 108-15) monitored by red-shifted green fluorescent protein

Activation of G protein-coupled receptors triggers translocation of certain proteins from cytoplasm to cell membrane located targets. One of these cytosolic proteins is phosducin (Phd) which has been described to compete with G protein-coupled receptor kinases for Gbetagamma dimers attached to the cell membrane, thereby attenuating desensitization of activated receptors. These features of protein redistribution prompted us to examine whether stimulation of membrane associated E-prostaglandin receptors coupled to Gs causes Phd to migrate towards the plasma membrane. We made use of enhanced green fluorescence protein (EGFP), a reporter protein, to follow redistribution of Phd both by means of confocal microscopy and biochemical techniques in living neuronal NG 108-15 hybrid cells challenged with prostaglandin E1 (PGE1). The cells were transiently transfected to express Phd fused to the C-terminus of EGFP, or to express EGFP only. Overexpression of the proteins is implied by FACS analysis as well as by western blot technique, and the functional integrity of EGFP-tagged Phd was confirmed by its ability to elevate cAMP accumulation. Time-lapse imaging of single living cells by means of confocal microscopy revealed that exposure to prostaglandin causes EGFP/Phd, which is evenly spread throughout the cell, to relocate towards the membrane within few minutes. Fluorescence associated with the cell nucleus displayed little rearrangement. The principle finding that prostaglandin triggers translocation of Phd from cytosol to the cell periphery was verified with membranes prepared from EGFP/Phd expressing cells. We found maximal concentrations of membrane associated fluorescent material 5 to 7 min upon prostaglandin exposure. The present study reports for living NG 108-15 hybrid cells that PGE1 stimulation causes cytosolic Phd to translocate towards the membrane, where it is believed to bind to G protein subunits such as Gbetagamma and Galphas.

A phosphorylation site in the ftz homeodomain is required for activity

The Drosophila homeodomain-containing protein Fushi tarazu (Ftz) is expressed sequentially in the embryo, first in alternate segments, then in specific neuroblasts and neurons in the central nervous system, and finally in parts of the gut. During these different developmental stages, the protein is heavily phosphorylated on different subsets of Ser and Thr residues. This stage-specific phosphorylation suggests possible roles for signal transduction pathways in directing tissue-specific Ftz activities. Here we show that one of the Ftz phosphorylation sites, T263 in the N-terminus of the Ftz homeodomain, is phosphorylated in vitro by Drosophila embryo extracts and protein kinase A. In the embryo, mutagenesis of this site to the non-phosphorylatable residue Ala resulted in loss of ftz-dependent segments. Conversely, substitution of T263 with Asp, which is also non-phosphorylatable, but which successfully mimics phosphorylated residues in a number of proteins, rescued the mutant phenotype. This suggests that T263 is in the phosphorylated state when functioning normally in vivo. We also demonstrate that the T263 substitutions of Ala and Asp do not affect Ftz DNA-binding activity in vitro, nor do they affect stability or transcriptional activity in transfected S2 cells. This suggests that T263 phosphorylation is most likely required for a homeodomain-mediated interaction with an embryonically expressed protein.

Phosducin expression in NG 108-15 hybrid cells enhances prostaglandin E1 stimulated adenylate cyclase activity

Phosducin (Phd), a cytosolic protein, has been proposed to compete with certain receptor kinases for Gbetagamma of heterotrimeric G proteins, and may inhibit GTPase activity of G alpha s. These suggestions together with the enhancing effect of Phd on odorant-induced cAMP accumulation let us assume a stimulatory action of the protein on intracellular signaling. Therefore, this investigation was designed to examine the excitatory effect of PGE1 on signal transmission in neuroblastoma x glioma hybrid cells (NG 108-15) overexpressing Phd. The neuronal cells stably expressing Phd were found to display a 3 to 4-fold increased sensitivity to PGE1 as compared to wild type cells, using cAMP accumulation as measure. Examination of membranes prepared from Phd-overexpressing cells revealed an elevated GTPase activity as indicated by the formation of 32Pi upon PGE1 challenge. This activity was inhibited by exogenous Phd. In addition, receptor independent stimulation of adenylate cyclase by forskolin reveals an increased formation of cAMP in Phd expressing cells, which is accompanied by an increased binding of [3H]forskolin. The findings let us propose that Phd elevates intracellular levels of functional G alpha s which accounts for the increased response to PGE1.

Molecular and cellular basis of addiction

Drug addiction results from adaptations in specific brain neurons caused by repeated exposure to a drug of abuse. These adaptations combine to produce the complex behaviors that define an addicted state. Progress is being made in identifying such time-dependent, drug-induced adaptations and relating them to specific behavioral features of addiction. Current research needs to understand the types of adaptations that underlie the particularly long-lived aspects of addiction, such as drug craving and relapse, and to identify specific genes that contribute to individual differences in vulnerability to addiction. Understanding the molecular and cellular basis of addictive states will lead to major changes in how addiction is viewed and ultimately treated.

Molecular mechanisms of opiate and cocaine addiction

Chronic administration of opiates or cocaine has been shown to alter the activity or expression of diverse types of cellular proteins in specific target neurons within the central nervous system. Prominent examples include signaling proteins, such as receptors, G proteins, second-messenger synthetic enzymes, and protein kinases. It is now increasingly possible to relate particular molecular adaptations to specific behavioral actions of drugs of abuse in animal models of addiction. In addition, recent work has focused on a role for transcription factors, and the associated alterations in gene expression, in mediating part of this long-lasting, drug-induced molecular and behavioral plasticity.